Adsorbate-induced surface stress: CO on Ni(100) and Ni(111)

Abstract

We have determined the surface stress induced by the room-temperature adsorption of carbon monoxide on the (100) and (111) surfaces of nickel by measuring the bending of a thin nickel crystal. While CO adsorption induces exclusively a compressive stress on Ni(111), the stress is tensile on Ni(100) at CO coverages below 0.2 ML. The tensile stress passes through a maximum of + 0.96 N/m at θ = 0.09 to become compressive for θ > 0.25 with a value of −0.54 N/m at θ = 0.5. The latter value is small compared with the stress produced by other, more strongly chemisorbed adsorbates on the same surface. As CO molecules predominantly occupy terminal binding sites on Ni(100) at 300 K, we must exclude a site conversion process in order to explain the stress-coverage dependence. The sign reversal of surface stress might be attributed to a coverage dependent variation in the net charge transfer between the metal surface and the adsorbate, involving an enhanced splitting of the bonding and antibonding 2π ∗ CO orbitals of neighboring molecules at higher adsorbate coverages.